Just looking at the CE pad, is the 256k one made properly? Bit difficult to see.

Other than that, presuming the roms are all good, which I would trust to be as its exxos roms, then with a multimeter check that things like 5v, 0v, ce or etc are all connected on each rom. Because it works in two chip mode I'd expect the lines to the cpu to be ok but maybe there is a problem with one of more sockets. Try cleaning them with IPA.

Other than that it's a case of metering them all out and looking for shorts or breaks. Possibly just a dry or corroded joint due to age.

Agree with the others on pin 14 of U63. Looking at the pic, A16 looks like it may be shorted to a small pad/via in the middle, and whilst I'm not 100% sure that it matters because the pic gets fuzzy when I zoom in, I'd be inclined to clean it up and re solder it a bit motre neatly.

Yes, you were all right : U63 pin 14 wasn't in the right place. Now it is.

And i have almost the same behavior. The difference is that it flickers very quickly at boot loop with an increasing number of bomb (4 then maybe 5 then maybe 6...). It's hard to see because it's very quick.